I had that same type of issue come up recently with chicks I sold as day olds. Honestly, there are so many things that could cause a little one to pass...even just the stress of moving can wreak havoc. I believe a lot of places have a 48 hour limit on replacing lost chicks. I think that's fair. After that, I mean, really? How do you know a child didn't squeeze it too hard? Or if the place is too warm/cold? The list goes on. But (like you, I'm sure) I want people with my chicks to be happy. It's a balancing act that, quite honestly (for me), depends a lot on how I am treated. Same goes with hatching eggs...I had a buyer tell me NONE were fertile. (They were shipped out of state, and there is a difference between fertile and viable.) I took offense to that. First, I know they're fertile. I've been hatching them! Second, if they can accuse you of selling a bad chick/infertile egg, I believe we have every right to question their brooding/incubation practices.
Now, after that rant, let me say that 9 out of 10 of my experiences with chicken people
have been great, and most who keep in contact just want advice and camaraderie. It USUALLY makes me very happy to be involved in the industry. It has also made me much softer as a buyer.